A number of optical sensors including respond to environmental changes by varying a property of an incident radiation. Some example properties that can be varies are transmittance, reflectance, absorbance or polarization of the incident radiation. As a result, systems for monitoring environmental effects using optical sensors such as Fabry-Peron elements and, even more so, Fiber Bragg Gratings (FBGs) have received considerable attention. FBGs are regions in optical fibers where the refractive index is varied in accordance to a certain pattern. Specifically, an FBG has a grating pitch Λ and an effective refractive index n that define a Bragg wavelength λB at which radiation is reflected by the FBG in accordance with the following equation:λB=2nΛ. 
Since pitch Λ and effective index n are very sensitive to changes in environmental effects such as temperature, pressure, strain, vibration, acoustics, etc., FBGs are effective in monitoring such changes. Consequently, numerous prior art systems teach to determine environmental changes directly from variations in Bragg wavelength λB using different systems and various operation modalities. For example, the reader is referred to U.S. Pat. No. 5,513,913 to Ball et al. for a description of an active operation mode and to U.S. Pat. No. 5,748,312 to Kersey for a description of time domain reflectometry with modulated signal. Of further interest are U.S. Pat. Nos. 5,684,297; 5,723,857; 5,798,521; 6,448,551 and 6,492,636.
Several prior art systems designed to measure stress and temperature variations in mechanical structures employ a fiber network with a number of FBGs at locations where these variations are to be monitored. These systems use a broadband light source for supplying radiation to the FBGs. The reflected portions of the radiation corresponding to the Bragg wavelengths of the FBGs are detected with the aid of a tunable filter. Davis et al. teaches an exemplary system of this kind in U.S. Pat. No. 5,818,585.
Unfortunately, the use of broadband light sources and tunable detectors incurs a number of disadvantages. First, since broadband sources intrinsically cover a wide emission spectrum they tend to consume a lot of energy. Second, such sources are unable to provide high radiation intensities within the narrow wavelength windows to which the Bragg wavelengths are confined. Hence, the intensities of the reflected portions tend to be low. Third, tunable filters tend to have wide and difficult to control passbands. Thus, they are not well suited for precisely monitoring Bragg wavelengths that are very narrow band. This situation is aggravated in cases where the Bragg wavelengths shift quickly and/or by small amounts due to correspondingly rapid or small variations of the environmental factors. Finally, systems using broadband sources tend to be complex and expensive.
To overcome some of the disadvantages associated with broadband sources U.S. Pat. No. 5,401,956 to Dunphy et al. teaches a diagnostic system for fiber grating sensor using tunable light sources. The system scans the light sources across a predetermined wavelength range and illuminates each sensor. This system can operate in a transmission or reflection mode. More recently, U.S. Pat. No. 6,204,920 to Ellerbrock et al. entitled Optical Fiber Sensor System teaches the use of one tunable light source, e.g., an LED and a tunable etalon, for delivering light to all sensors. Still more recently, U.S. Pat. No. 6,417,507 to Malvern et al. entitled “Modulated Fibre Bragg Grating Strain Gauge Assembly for Absolute Gauging of Strain” discloses the use of tunable light sources and frequency modulation to determine absolute direction and magnitude of strain from a ratio of reflected intensity values 1f:2f, where f is the modulation frequency.
Despite these advances, problems associated with precise monitoring of light reflected at Bragg wavelengths that are very narrow band and fluctuate due to environmental perturbations and system instabilities remain.